The quantile scale should ignore null, too.

d3.js

@@ -7703,9 +7703,7 @@
     }
     scale.domain = function(x) {
       if (!arguments.length) return domain;
-      domain = x.filter(function(d) {
-        return !isNaN(d);
-      }).sort(d3_ascending);
+      domain = x.filter(d3_number).sort(d3_ascending);
       return rescale();
     };
     scale.range = function(x) {

src/scale/quantile.js

 import "../arrays/ascending";
 import "../arrays/bisect";
 import "../arrays/quantile";
+import "../math/number";
 import "scale";
 
 d3.scale.quantile = function() {
@@ -24,7 +25,7 @@ function d3_scale_quantile(domain, range) {
 
   scale.domain = function(x) {
     if (!arguments.length) return domain;
-    domain = x.filter(function(d) { return !isNaN(d); }).sort(d3_ascending);
+    domain = x.filter(d3_number).sort(d3_ascending);
     return rescale();
   };
 

test/scale/quantile-test.js

@@ -30,7 +30,7 @@ suite.addBatch({
       assert.deepEqual(x.domain(), [3, 6, 7, 8, 8, 10, 13, 15, 16, 20]);
     },
     "non-numeric domain values are ignored": function(quantile) {
-      var x = quantile().domain([6, 3, NaN, undefined, 7, 8, 8, 13, 20, 15, 16, 10, NaN]);
+      var x = quantile().domain([6, 3, NaN, undefined, 7, 8, 8, 13, null, 20, 15, 16, 10, NaN]);
       assert.deepEqual(x.domain(), [3, 6, 7, 8, 8, 10, 13, 15, 16, 20]);
     },
     "quantiles returns the inner thresholds": function(quantile) {